Audio frequency limiter network



P 1944- M. KATZIN AUDIO FREQUENCY LIMITER NETWORK Filed March 11. 1941 100,0'00 Ge/o RES/37:02 -B.

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maven-i INVENTOR I u v A'YITORNEY I resistors.

Referring now in detail to Fig. l of the drawing, theinput circuit is connected to an audio fre- Patented Sept. 5, 1944 AUDIO FREQUENCY LIMITEB, NETWORK Martin Katzin, Riverhead, N. Y., assignor to Radio Corporatign of America, a corporation o;

Delaware Application March 11, 1941, Serial No. 382,732

2 Claims. 7 (01. 17844) This invention relates to a new and useful audio frequency limiter network.

An object of the present invention is the limitation of the amplitude of audio frequency signals to certain predetermined limits.

Still another object of the present invention is to prevent signal amplitudes from reaching dangerous levels in measuring circuits and thereby damaging the measuring instruments.

A feature of this invention is the proper arrangement and choice .of a grid leak resistor in an audio frequency limiter circuit whereby paralyzing efiects of excess amplitudes in radio receiving circuits may be avoided.

The present invention is particularly useful where it is desired to read weak telegraph signals in the presence of crashes of static and other noise of considerable amplitude. By passing the audio frequency signal as received through a limiter,constructed in accordance with the principles of the present invention, the paralyzing eflectsof excess amplitudes may be avoided.

Audio frequency circuits including frequency limiting devices are well-known in th prior art, however, I have found a very curious effect is obtained when different-values of grid leak resistance areinserted in the audio frequency circuits, as will.

be explained in more detail by the accompanying drawing, in which:

Fig. 1 is a circuitdiagram' of an audio frequen'cy circuit of this invention; and i Fig. 2 is a'curve showing the results obtained in the circuit, of Fig. 1, using different values of qucncy transformer l, which include input primary 2 and secondary 3. The outside ends-of secondary 3 areconnected to the input grids 4 and 5 of electron discharge devices 6 and lof the 606 triple-grid detector amplifier type. The

very satisfactory response in the output transformer ID. The screen grid of tubes 6 and 1 are connected together and thence to a potential divider network, including resistances I5, I 6, l1 and I8 and bypassed to'ground by means of a condenser l9. The suppressor grids are connected together and to the cathodes which are connected to a variable tap on resistor l5. Thus, the cathodes are maintained positive with respect to ground by anamount depending on the proportion of resistor l5 between the tap and ground to the resistance of the whole of potential divider networks l5, l6, I1 and I8. A double pole switch 20 is provided to simultaneously connect or disconnect the high potential source and the filament supply source from the' circuit. e

As mentioned above, I have found a very curious efi'ect when difierent values of grid leak resistor I 5 are employed in the circuit. The curves shown by Fig. 2 give the characteristic obtained for employing four different value for the resistor i6.

, It will be seen that the inclusion of the .resistance increases the steepness of the initial rise con-' siderably, but that too high a value of resistance produces a pronounced hump in thecharacteristic. With a proper value of resistance, this hump is eliminated. The curve A employ a grid resistance of 50,000 ohms and shows a preferred value of grid leak resistance which represents almost the optimum value. A slightly lower value, perhaps somewhere between 40,000 and 50,000

ohms, would probably eliminate the very slight humpwhich still remains atAl. The curve at B indicates that when a grid resistor of 100,000

ohms is employed, the hump BI is greater. Curve C indicates the use of a l-megohm resistor having a very pronounced hump at Cl. Curve D repre I sents zero grid resistance and is shown to give the plates 8 and 9' of the 'ele'ctrondischarge devices are connected 'to the primary of output transformer-l0 having a primary II and secondary I2, the outside of the secondary I2 being connected to any suitable output. The midpoint of winding l I is connected to the positive side of the filament supply source. Thus, with tubes of the type specitied the applied plate voltage is of the order of 6.3 volts. The midp'oint tap l3 ofinput transformer l is connected to a point of potential which ls-lower with respect tothe grids of electron discharge devices 6 and I by having connected therebetween a resistor 14 of which I have found the values between 4 0,000 to 50,000 ohms, to give a comparative differences between theselected values of grid resistors. The value of resistors shown in the curves of Fig. 2 are with-the use of 606 triple-grid detector amplifier tubes. Other values 7 of the circuit arrangement are 6,500 ohms for re- .sistors l5and l6 and 17,000 ohms for resistors I I1 and It. 'In a particular arrangement as constructed, a filament dropping resistor 2| was found necessary since the filament supply voltage was higher than the rated filament voltage for tubes 6 and l. The bypass condenser I9 is preferably in the order of one micro-farad. While this invention has been described as employing a certain value of grid leak resistance, for

' the tubes 6 and I and transformers l and Ill, it is to bedistinctly understood that these values may change slightly for different component parts 4 screen grid, an'input circuit including a trans-I ode,

former winding and a resistor connected in series between said control grid and ground, an output circuit coupled to said plate, means for applying a low positive potential with respect toground to said plate whereby the amplitude of signals applied to said input circuit and; appearing in said output circuit is limited to a predetermined value, said resistor I in said input circuit having such value that said predetermined value is attained for lower input levels than in-it's" absence and that the amplitude is maintained substantially constant for greater signal input levels.

- ground 2. A limiter circuit including a pair of thermionic discharge devices, each having at least a catha control grid, a screen grid and a plate, means for applying a low positive potential with respect to ground to said cathodes,;means forapplying a high positive potential with respect to to said screen grids, an input circuit including a transformer winding and a resistor connected in series between said control .g'rid and ground, an output circuit coupled tosaid plate,

'means for applying a low positive potential with respect to ground to said plate whereby the .amplitude oi signals applied to said input circuit and appearing in said output circuit is limited to a predetermined value, said resistor in said input circuit having such value that said predetermined value is attained for lower input levels than'in its absence and that the amplitude is maintained substantially constant for greater signal input levels, said value being of the order of 50,000 ohms. 1

' MARTIN KATZIN. 

